To get a really full/fat sound in the mix try the following:
100% left rhythm 1
100% right rhythm 2
(use the same guitar sound with harmony variations on each side - eg some notes up 5th or octave on either side to create a full chord structure - works very well if you're using power chords progressions)
centre rhythm track 3
(use a different guitar sound and keep lowest basic power chords)
50% right harmony 1
50% left harmony 2
(works well with melodic progressions played over a rhythm bed)
centre bass track
(aim for a wide/fat sound)
25/30% left lead 1
25/30% right lead 2
(good for counter solos or harmony solos)
centre lead
(main voice guitar solo)
Give it a go, it makes a huge difference to the overall sound. Most rock/metal recordings have a minimum of 3 rhythm tracks to fill things out. Most important thing here is to make sure that you play as tight as possible. If one guitar lags or misses a few notes it will stand out more so than if everything was mixed in the centre, the good thing about that is you will have to scrutinise your playing more, so you end up with better technique.